Communication transfer apparatus and communication transfer method

ABSTRACT

A communication transfer apparatus and a communication transfer method can transfer communications at low cost without the need of requesting a global IP network to switch any port number. The local internet protocol address of the origin terminal of transfer described in a record relating to a transfer out of the records of the masquerade table that is utilized for an internet protocol masquerade is rewritten as the local internal protocol address of the destination terminal of transfer, while maintaining the global port number of the record.

This application is a division of application Ser. No. 11/012,218, filedDec. 16, 2004, now pending, based on Japanese Patent Application No.2003-419635, filed Dec. 17, 2003, by Hidehiko FUJIWARA and Naoki MORI,which is incorporated herein by reference in their entirety. Thisapplication claims only subject matter disclosed in the parentapplication and therefore presents no new matter.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a communication transfer apparatus and acommunication transfer method for transferring communications. Moreparticularly, the present invention relates to a communication transferapparatus and a communication transfer method for switching a terminalcommunicating with another terminal by way of the global IP (internetprotocol) network to some other terminal belonging to the local IPnetwork to which the first terminal belongs.

2. Related Background Art

As the development of Internet-related technologies, IP telephony thatutilizes the Internet is becoming very popular. With IP telephony, avoice communication can proceed by means of RTP (real-time transportprotocol) or some other protocol once a communication line isestablished by signaling by means of SIP (session initiation protocol),H.323 protocol or some other protocol.

Technological documents that describe the conventional art relating tothe present invention include JP 11-205475 A, JP 2000-286882 A and JP2003-46665 A.

However, the IP telephony that is becoming popular has a prerequisitethat a single IP telephone terminal is connected to a single router thatis installed in an office or a home.

In other words, when there is an IP telephone call, the routerestablishes a corresponding relationship between the combination of theglobal IP address and the global port number and the combination of thelocal IP address and the local port number of the called terminal bymeans of the technique of NAT (network address translation) or IPmasquerade and the corresponding relationship continues as long as theIP telephone communication proceeds.

Therefore, if more than one IP telephone terminals are connected to therouter, it may be conceivable to use either of the following twotechniques in order to transfer the IP telephone communication of an IPtelephone terminal to another IP telephone terminal.

The first one is to request the global network or the other end of theline to change the global port number that corresponds to the local IPaddress and the local port number of the IP telephone terminal beforethe transfer to the global port number that corresponds to the local IPaddress and the local port number of the IP telephone terminal of thedestination of transfer.

However, any existing Internet service provider cannot accommodate sucha request and any existing router cannot accommodate such a requesteither.

The second conceivable technique is to terminate the IP packets to beused with the other end of the line for IP telephone communication andinstall a particular device for IP telephone communications in the localIP network.

However, while an IP telephone communication can be realized between twoterminals on a peer to peer basis, installing such a device for thepurpose of termination and redistribution will again raise the cost thathas been reduced once.

The above-identified problem arises not only in IP telephonecommunications but also in visual phone communications, television gamesand communications using messengers.

SUMMARY OF THE INVENTION

In view of the above described circumstances, it is therefore an objectof the present invention to provide a communication transfer apparatusand a communication transfer method that eliminate the necessity ofrequesting the global IP network (Internet) to switch the port numberand allow to transfer communications at low cost.

According to a first aspect of the present invention, there is provideda communication transfer apparatus comprising rewriting means forrewriting a local internet protocol address of an origin terminal oftransfer described in a record relating to the transfer out of recordsof a masquerade table which is utilized for an internet protocolmasquerade into a local internet protocol address of a destinationterminal of the transfer, while maintaining a global port number of therecord.

The communication transfer apparatus according to the first aspect mayfurther comprise: local internet protocol address of origin of transferdetecting means for detecting a local internet protocol address of anorigin of the transfer assigned to the origin terminal of the transfer;and local port number of origin of transfer detecting means fordetecting a local port number of the origin of the transfer utilized bythe origin terminal of the transfer, wherein said rewriting meansidentifies the record relating to the transfer by the local internetprotocol address of the origin of the transfer detected by said localinternet protocol address of origin of transfer detecting means and thelocal port number of the origin of the transfer detected by said localport number of origin of transfer detecting means.

The communication transfer apparatus according to the first aspect mayfurther comprise: an extension number versus local internet protocoladdress correspondence table storing a relationship between an extensionnumber assigned to each terminal and a local internet protocol addressassigned to the terminal; and extension number of origin of transferdetecting means for detecting the extension number of the origin of thetransfer assigned to the origin terminal of the transfer, wherein saidlocal internet protocol address of origin of transfer detecting meansdetects a local internet protocol address corresponding to the extensionnumber of the origin of the transfer detected by said extension numberof origin of transfer detecting means from said extension number versuslocal internet protocol address correspondence table as the localinternet protocol address of the origin of the transfer.

The communication transfer apparatus according to the first aspect mayfurther comprise: local internet protocol address of destination oftransfer detecting means for detecting a local internet protocol addressof a destination of transfer assigned to the destination terminal of thetransfer, wherein said rewriting means rewrites the local internetprotocol address of the origin terminal of the transfer described in therecord relating to the transfer into a local internet protocol addressof the destination of transfer detected by said local internet protocoladdress of destination of transfer detecting means.

The communication transfer apparatus according to the first aspect mayfurther comprise: an extension number versus local internet protocoladdress correspondence table storing a relationship between an extensionnumber assigned to each terminal and a local internet protocol addressassigned to the terminal; and extension number of destination oftransfer detecting means for detecting the extension number of thedestination of the transfer assigned to the destination terminal of thetransfer, wherein said local internet protocol address of destination oftransfer detecting means detects a local internet protocol addresscorresponding to the extension number of the destination of transferdetected by said extension number of destination of transfer detectingmeans from said extension number versus local internet protocol addresscorrespondence table as the local internet protocol address of thedestination of transfer.

In the communication transfer apparatus according to the first aspect,said rewriting means may realize the rewrite by erasing the recordrelating to the transfer and adding a record describing a global portnumber identical with the global port number described in the erasedrecord and the local internet protocol address of the destinationterminal of the transfer to said masquerade table.

The communication transfer apparatus according to the first aspect mayfurther comprise: local internet protocol address of origin of transferdetecting means for detecting a local internet protocol address of anorigin of the transfer assigned to the origin terminal of the transfer;local port number of origin of transfer detecting means for detecting alocal port number of the origin of the transfer utilized by the originterminal of the transfer; a replica of said masquerade table; andretrieving means for retrieving a global port number corresponding tothe local internet protocol address of the origin of the transferdetected by said local internet protocol address of origin of transferdetecting means and the local port number detected by said local portnumber of origin of transfer detecting means from the replica, whereinsaid rewriting means identifies the record relating to the transfer bythe global port number retrieved by said retrieving mans and write theglobal port number detected by said retrieving means into the addedrecord.

According to a second aspect of the present invention, there is provideda communication transfer apparatus comprising: presence informationstoring means for storing information on presence or absence of a userat each terminal and information on priorities of terminals for eachuser; called terminal detecting means for detecting a called terminal;presence determining means for determining presence or absence of a userat the called terminal detected by said called terminal detecting meansby referring to said presence information storing means; destination oftransfer identifying means for identifying a terminal having a nextpriority by referring to said presence information storing means if saidpresence determining means determines absence of a user at the calledterminal detected by said called terminal detecting means; and recordcreating means for creating a record describing a local internetprotocol address of the terminal identified by said destination oftransfer identifying means as well as a global port number and localport number to be used for communication to a masquerade table utilizedfor an internet protocol masquerade.

According to a third aspect of the present invention, there is provideda communication transfer apparatus comprising: priority storing meansfor storing information on priorities of terminals for each user; calledterminal detecting means for detecting a called terminal; presencedetermining means for determining execution or non-execution of ahooking off operation at the called terminal detected by said calledterminal detecting means within a predetermined period of time;destination of transfer identifying means for identifying a terminalhaving a priority next to a priority of the called terminal by referringto said priority storing means if said presence determining meansdetermines non-execution of the hooking off operation at the calledterminal detected by said called terminal detecting means within thepredetermined period of time; and record creating means for creating arecord describing a local internet protocol address of the terminalidentified by said destination of transfer identifying means as well asa global port number and local port number to be used for communicationto a masquerade table utilized for an internet protocol masquerade.

Thus, according to the invention, it is no longer necessary to requestthe global IP network (Internet) to switch the port number so that it ispossible to transfer communications at low cost.

Additionally, according to the invention, an effect similar to that ofrewriting a masquerade table can be obtained by erasing a record andadding another record, referring to a replica of the masquerade table ifit is not possible to acquire information therefrom for identifying therecord of the masquerade table to be rewritten and if it is not possibleto directly rewrite the record of the masquerade table.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a schematic block diagram of a communication system realized bymeans of an embodiment of the present invention, illustrating theconfiguration thereof;

FIG. 2 is a schematic block diagram of the own office router and thecall control device of a first embodiment of the present invention,illustrating the configuration thereof;

FIG. 3 is a schematic illustration of the masquerade table of FIGS. 2and 11, showing the configuration thereof;

FIG. 4 is a schematic illustration of the extension number versus localIP address correspondence table of FIGS. 2 and 11, showing theconfiguration thereof;

FIG. 5 is a schematic illustration of the first format of information tobe stored in the presence information storage section of FIGS. 2 and 11;

FIG. 6 is a schematic illustration of the second format of informationto be stored in the presence information storage section of FIGS. 2 and11;

FIG. 7 is a flow chart of the operation of the call control device andother related components of the first embodiment of the presentinvention at the time of starting a communication;

FIG. 8 is a flow chart of the operation of the call control device andother related components of the first embodiment of the presentinvention at the time of a transfer;

FIG. 9 is a flow chart of the operation that is conducted by using thepresence information of the call control device and other relatedcomponents and by using the first method at the time of starting acommunication of the first embodiment of the present invention;

FIG. 10 is a flow chart of the operation that is conducted by using thepresence information of the call control device and other relatedcomponents and by using the second method at the time of starting acommunication of the first embodiment 1 of the present invention;

FIG. 11 is a schematic block diagram of the own office side router andthe call control device of a second embodiment of the present invention,illustrating the configuration thereof;

FIG. 12 is a flow chart of the operation of the call control device andother related components of the second embodiment of the presentinvention at the time of starting a communication;

FIG. 13 is a flow chart of the operation of the call control device andother related components of the second embodiment of the presentinvention at the time of a transfer;

FIG. 14 is a flow chart of the operation that is conducted by using thepresence information of the call control device and other relatedcomponents and by using the first method at the time of starting acommunication of the second embodiment 2 of the present invention; and

FIG. 15 is a flow chart of the operation that is conducted by using thepresence information of the call control device and other relatedcomponents and by using the second method at the time of starting acommunication of the second embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now, the present invention will be described by referring to theaccompanying drawings that illustrate the preferred embodiments ofcarrying out the invention.

[Embodiment 1]

FIG. 1 a schematic block diagram of a communication system realized bymeans of an embodiment of the present invention, illustrating theconfiguration thereof. An own office has an own office side router 101,a call control device 102 and first through N-th terminals including afirst terminal 103-1, a second terminal 103-2, . . . , an N-th terminal103-N. The own office side router 101, the call control device 102 andthe first terminal 103-1, the second terminal 103-2, . . . , the N-thterminal 103-N are connected to a local IP network 104. The call controldevice 102 is connected to the first terminal 103-1, the second terminal103-2, . . . , the N-th terminal 103-N by way of respective controllines 105-1, 105-2, . . . , 105-N.

The partner office has a partner office side server 112 and a partneroffice side terminal 113.

The own office side router 101 and the partner office side server 112are connected to each other by way of a global IP network (the Internet)111. Therefore, the partner office side terminal 113 is adapted tocommunicate with any of the first terminal 103-1, the second terminal103-2, . . . , the N-th terminal 103-N by way of the partner office sideserver 112, the global IP network 111 and the own office side router101.

As shown in FIG. 2, the own office side router 101 is provided with amasquerade table 101-1, an IP masquerade section 101-2 and a negotiationsection 101-3. The own office side router 101 additionally hasfunctional sections (not shown) that an ordinary router is normallyprovided with.

As shown in FIG. 3, the masquerade table 101-1 stores one or morerecords each describing a relationship between a combination of a globalIP address and a global port number and a combination of a local IPaddress and a local port number. In the example of FIG. 3, the firstrecord describes that the combination of global IP address 10.10.20.1and global port number 50001 corresponds to the combination of local IPaddress 192.168.0.11 and local port number 60001. In this embodiment, arecord may be added to, modified in or erased from the masquerade table101-1 appropriately whenever necessary. The global IP address may beomitted from the masquerade table 101-1. An IP masquerade can berealized even if the global IP address is omitted from the masqueradetable 101-1, because it is possible to know which global port numbercorresponds to which combination of a local IP address and a local portnumber.

The IP masquerade section 101-2 conducts an IP masquerade by referringto the masquerade table 101-1.

At the time of signaling, the negotiation section 101-3 negotiates withthe partner office side server 112. As a result of the negotiation, thepartner office side server 112 determines the global port number to beutilized typically by the UDP/IP for RTP packet transmission or someother transmission and notifies the determined global port number torecord adding section 102-5. The determined global port number is notchanged as long as the communication continues.

As shown in FIG. 2, the call control device 102 has an extension numberdetecting section 102-1, an extension number versus local IP addresscorrespondence table 102-2, a local IP address retrieving section 102-3,a local port number storage section 102-4, a record adding section102-5, a terminal interface 102-6, a local IP address assigning section102-7, an origin-of-transfer-related information detecting section102-8, a record retrieving section 102-9, a destination terminaldesignation receiving section 102-10, a transfer hold receiving section102-11, a release terminal detecting section 102-12, a post-transferlocal IP address detecting section 102-13, a recording rewriting section102-14, a record erasing section 102-15, a presence informationacquiring section 102-16 and a presence information storage section102-17.

When the partner office side terminal 113 designates the directtelephone number of a particular one of the first through N-th terminals103-1 through 103-N, the extension number detecting section 102-1detects the extension number of the terminal on the basis of the directtelephone number. Then, the extension number detection section 102-1requests the terminal interface 102-6 to ring a call-in bell of theterminal, using the extension number. When, on the other hand, thepartner office side terminal 113 calls the representative telephonenumber, the extension number detecting section 102-1 requests theterminal interface 102-6 to ring a call-in bells of all the terminals.Then, the extension number detecting section 102-1 is notified of theextension number of the terminal that is hooked off first by theterminal interface 102-6.

As shown in FIG. 4, the extension number versus local IP addresscorrespondence table 102-2 stores one or more records each describing arelationship between an extension number assigned to a terminal and alocal IP address assigned to the terminal.

The local IP address retrieving section 102-3 receives an extensionnumber from the extension number detecting section 102-1 and retrievesthe local IP address corresponding to the received extension number fromthe extension number versus local IP address correspondence table 102-2.The retrieved local IP address then becomes the local IP address of theterminal that is to start a communication.

The local port number storage section 102-4 holds the local port numberthat corresponds to the type of the coming communication.

At the time of signaling, the record adding section 102-5 receives theglobal IP address and the global port number to be used for the comingcommunication from the negotiation section 101-3. Note, however, that itis not necessary for the record adding section 102-5 to receive theglobal IP address from the negotiation section 101-3 when the global IPaddress of the own office side router 101 is known or when no field isprovided for global IP addresses in the masquerade table 101-1.Additionally, at the time of signaling, the record adding section 102-5receives the local IP address to be used for the coming communication(the local IP address of the terminal to be used for the comingcommunication) from the local IP address retrieving section 102-3.Furthermore, at the time of signaling, the record adding section 102-5receives the local port number to be used for the coming communicationfrom the local port number storage section 102-4. Then, the recordadding section 102-5 adds a record describing the global IP address, theglobal port number, the local IP address and the local port number ithas received to the masquerade table 101-1. Note, however, that theadded record describes only the global port number, the local IP addressand the local port number when no field is provided for global IPaddresses in the masquerade table 101-1.

The record adding section 102-5 may determine the local port number bynegotiating with the terminal with which it is going to communicate. Therecord adding section 102-5 may receive a designated local port numberalthough the record adding section 102-5 has the initiative of thenegotiation in hand.

The terminal interface 102-6 communicates with the first terminal 103-1,the second terminal 103-2, . . . , the N-th terminal 103-N for thepurpose of controlling the call.

The local IP address assigning section 102-7 receives the local IPaddress of the terminal that is to be newly added as extension from thisterminal, adds a new extension number that is generated internally andadds a record describing the new local IP address and the new extensionnumber to the extension number versus local IP address correspondencetable 102-2.

The origin-of-transfer-related information detecting section 102-8detects the extension number or the local IP address of the terminalrequesting transfer of communication. When theorigin-of-transfer-related information detecting section 102-8 detectsthe extension number of the terminal requesting transfer ofcommunication, the origin-of-transfer-related information detectingsection 102-8 retrieves the local IP address corresponding to theextension number from the extension number versus local IP addresscorrespondence table 102-2. Additionally, the origin-of-transfer-relatedinformation detecting section 102-8 detects the type of thecommunication of which transfer is requested or the local port numberthat is used for the communication of which transfer is requested. Ifthe origin-of-transfer-related information detecting section 102-8detects the type of the communication of which transfer is requested, itretrieves the local port number corresponding to the type of thecommunication from the local port number storage section 102-4. If, forexample, the type of communication is limited to IP telephony and thelocal port number is fixed, it does not have to retrieve the local portnumber but have only to retain the local port number. Theorigin-of-transfer-related information detecting section 102-8 mayalternatively inquire the local port number being used for communicationto the origin terminal of the transfer.

The record retrieving section 102-9 receives the local IP address andthe local port number of the origin of transfer from theorigin-of-transfer-related information detecting section 102-8 andretrieves the record describing the received local IP address and localport number from the masquerade table 101-1. Then, the record retrievingsection 102-9 notifies the record rewriting section 102-14 ofinformation for identifying the retrieved record (including the recordnumber).

Two methods can be used for transfer of communication as describedbelow. The first one is that the origin of transfer designates thedestination of transfer. More specifically, the origin terminal oftransfer holds the communication and notifies to the designateddestination of transfer by way of extension that the origin terminal oftransfer transfers the communication to the designated destination oftransfer. Then, the origin terminal of transfer selects the designateddestination of transfer after disconnecting the extension. Thecommunication resumes as the designated destination of transfer releasethe communication that has been held. The second method is that theorigin of transfer does not designate the destination of transfer. Morespecifically, the origin terminal of transfer holds the communicationand the terminal that firstly releases the communication that has beenheld becomes the destination terminal of transfer.

The destination terminal designation receiving section 102-10 is adaptedto operate for the first transfer method and receive the extensionnumber of the destination terminal of transfer from the origin terminalof transfer.

The transfer hold receiving section 102-11 and the release terminaldetecting section 102-12 are adapted to operate for the second transfermethod. The transfer hold receiving section 102-11 receives a hold ofcommunication by the origin terminal of transfer and notifies therelease terminal detecting section 102-12 of that the communication isin a held state. The release terminal detecting section 102-12 detectsthe terminal that is firstly hooked off first as a release terminal.

The post-transfer local IP address detecting section 102-13 receives theextension number of the destination terminal of transfer from thedestination terminal designation receiving section 102-10 when the firsttransfer method is used whereas the post-transfer local IP addressdetecting section 102-13 receives the extension number of thedestination terminal of transfer from the release terminal detectingsection 102-12 when the second transfer method is used. Then, thepost-transfer local IP address detecting section 102-13 retrieves thelocal IP address that corresponds to the received extension number fromthe extension number versus local IP address correspondence table 102-2.

The record rewriting section 102-14 receives information for specifyingthe record to be rewritten out of the records of the masquerade table101-1 and the local IP address to be written to the record to berewritten (the local IP address of the destination terminal of transfer)respectively from the record retrieving section 102-9 and thepost-transfer local IP address detecting section 102-13 and rewrites therecord to be rewritten among the records of the masquerade table 101-1so as to replace the local IP address in the record by the local IPaddress of the destination terminal of transfer. As a result of thisrewriting, the transfer of communication is realized.

In principle, it is not necessary to rewrite the local port number ifthe local port number is determined for the type of the communicationthat is taking place. However, if it is necessary to change the localport number, the record rewriting section also rewrites the local portnumber by the new local port number on the record to be rewritten amongthe records of the masquerade table 101-1.

It is necessary to change the local port number, for example, when thelocal port number is designated by the destination terminal of transferin the negotiation with the destination terminal of transfer or when thedefault local port number that is to be used by the destination terminalof transfer is occupied at present.

The record erasing section 102-15 detects the extension number or thelocal IP address of the terminal that has terminated a communication. Ifthe record erasing section 102-15 detects the extension number of theterminal that has terminated a communication, the record erasing section102-15 retrieves the local IP address that corresponds to the extensionnumber from the extension number versus local IP address correspondencetable 102-2. The record erasing section 102-15 also detects the type ofthe communication that has been terminated or the local port number usedfor the communication that has been terminated. If the record erasingsection 102-15 detects the type of the communication that has beenterminated, the record erasing section 102-15 retrieves the local portnumber that corresponds to the type of the communication from the localport number storage section 102-4. If, for example, the type ofcommunication is limited to IP telephony and the local port number isfixed, the record erasing section 102-15 does not have to detect norretrieve the local port number but only has to retain the local portnumber. The record erasing section 102-15 then erases the record thatdescribes the detected or retrieved local IP address and the detected orretrieved local port number from the masquerade table 101-1. By doingso, the unnecessary port is closed to improve the security and preventthe unnecessary record from remaining in the masquerade table 101-1.

The presence information storage section 102-17 stores information onthe current presence or absence of the user at each terminal. It alsostores information on the priority of the available terminals for eachuser. Information for identifying a terminal may be the extension numberor the local IP address of the terminal. If a terminal is identified bythe extension number thereof, the information stored in the presenceinformation storage section 102-17 will be such as illustrated in FIG.5. If, on the other hand, a terminal is identified by the local IPaddress thereof, the information stored in the presence informationstorage section 102-17 will be such as illustrated in FIG. 6.

The presence information acquiring section 102-16 acquires theinformation to be stored in the presence information storage section102-17 from each terminal by way of the terminal interface 102-6.Alternatively, the presence information acquiring section 102-16 mayacquire the information to be stored in the presence information storagesection 102-17 from each terminal by way of local IP network 104.

If a terminal is identified by the extension number thereof by way ofthe presence information storage section 102-17, the extension numberdetecting section 102-1 may operate in a manner as described below,using presence information, in addition to the above describedoperation.

When a call specifying a particular terminal is received and it is madeclear from the information stored in the presence information storagesection 102-17 that the user is currently absent at the terminal, theterminal having the second degree of priority is selected as terminalfor receiving the call out of the terminals available to the user andthe extension number of the terminal is delivered to the local IPaddress retrieving section 102-3. This operation is repeated until oneof terminals concerned is hooked off or the call gets to the terminalhaving the lowest degree of priority.

When a call specifying a particular terminal is received and it is madeclear from the information stored in the presence information storagesection 102-17 that the user is supposed to be currently present at theterminal but the terminal is not hooked off within a predeterminedperiod of time (e.g., 30 seconds), the terminal having the second degreeof priority is selected as the terminal for receiving the call out ofthe terminals available to the user and the extension number of theterminal is delivered to the local IP address retrieving section 102-3.This operation is repeated until one of terminals concerned is hookedoff or the call gets to the terminal having the lowest degree ofpriority.

If a terminal is identified by the local IP address thereof in thepresence information storage section 102-17, the local IP addressretrieving section 102-3 may operate in a manner as described below,using presence information, in addition to the above describedoperation.

When a call specifying a particular terminal is received and, as aresult of receiving the extension number from the extension numberdetecting section 102-1 and retrieving the local IP addresscorresponding to the extension number from the extension number versuslocal IP address correspondence table 102-2, it is made clear from theinformation stored in the presence information storage section 102-17that the user is currently absent at the terminal, the terminal havingthe second degree of priority is selected as terminal for receiving thecall out of the terminals available to the user and the local IP addressof the terminal is delivered to the record adding section 102-5. Thisoperation is repeated until one of terminals concerned is hooked off orthe call gets to the terminal having the lowest degree of priority.

When a call specifying a particular terminal is received and, as aresult of receiving the extension number from the extension numberdetecting section 102-1 and retrieving the local IP addresscorresponding to the extension number from the extension number versuslocal IP address correspondence table 102-2, it is made clear from theinformation stored in the presence information storage section 102-17that the user is supposed to be currently present at the terminal butthe terminal is not hooked off within a predetermined period of time(e.g., 30 seconds), the terminal having the second degree of priority isselected as the terminal for receiving the call out of the terminalsavailable to the user and the local IP address of the terminal isdelivered to the record adding section 102-5. This operation is repeateduntil one of terminals concerned is hooked off or the call gets to theterminal having the lowest degree of priority.

Now, the operation of the own office side router 101 and that of thecall control device 102 at the time of starting a communication will bedescribed below by referring to FIG. 7.

Firstly, the negotiation section 101-3 negotiates with the partneroffice side server 112 to determine the global port number to be usedfor the communication (Step S201). Then, the extension number detectingsection 102-1 detects the extension number of the terminal to be usedfor the communication (Step S202). Then, the local IP address retrievingsection 102-3 retrieves the local IP address that corresponds to theextension number detected in Step S202 from the extension number versuslocal IP address correspondence table 102-2 (Step S203). The local IPaddress is that of the terminal to be used for the communication.Thereafter, the record adding section 102-5 determines the local portnumber to be used for the communication (Step S204). Then, the recordadding section 102-5 adds a record describing the correspondingrelationship among the global port number determined in Step S201, thelocal IP address retrieved in Step S203 and the local port numberdetermined in Step S204 to the masquerade table 101-1 (Step S205).

Now, the operation of the own office side router 101 and that of thecall control device 102 at the time of transferring the communicationwill be described below by referring to FIG. 8.

Firstly, the origin-of-transfer-related information detecting section102-8 detects the extension number of the origin terminal of transfer(Step S211). Then, the origin-of-transfer-related information detectingsection 102-8 retrieves the local IP address that corresponds to theextension number detected in Step S211 from the extension number versuslocal IP address correspondence table 102-2 (Step S212). Alternatively,the origin-of-transfer-related information detecting section 102-8 maydirectly detect the local IP address of the origin terminal of transferin place of the detecting operation of Steps S211 and S212 (Step S213).Then, the origin-of-transfer-related information detecting section 102-8detects the local port number that is being used for the communication(Step S214). Thereafter, the record retrieving section 102-9 retrievesthe information for identifying the record describing the local IPaddress retrieved in Step S212 or in Step S213 and the local port numberdetected in Step S214 from the masquerade table 101-1 (Step S215). Therecord is to be rewritten.

Subsequently, the destination terminal designation receiving section102-10 or the release terminal detecting section 102-12 detects theextension number of the destination terminal of transfer (Step S216).Then, the post-transfer local IP address detecting section 102-13retrieves the local IP address that corresponds to the extension numberof the destination of transfer detected in Step S216 from the extensionnumber versus local IP address correspondence table 102-2 (Step S217).Alternatively, the destination terminal designation receiving section102-10 or the release terminal detecting section 102-12 may directlydetect the local IP address of the destination terminal of transfer inplace of the detecting operation of Steps S216 and S217 (Step S218).Then, the record rewriting section 102-14 determines the post-transferlocal port number (Step S219). Then, the record rewriting section 102-14rewrites the record identified by the identifying information asretrieved in Step S215 so as to replace the local IP address and thelocal port number in the record respectively by the local IP addressretrieved in Step S217 or detected in Step S218 and the local portnumber determined in Step S219 (Step S220).

Next, the processing operation of the own office side router 101 andthat of the call control device 102 at the time of automatic transferaccording to the presence information and by means of the abovedescribed first method will be described by referring to FIG. 9.

Firstly, the negotiation section 101-3 negotiates with the partneroffice side server 112 to determine the global port number to be usedfor the communication (Step S201). Then, the extension number detectingsection 102-1 determines whether an extension number is designated (StepS242). If there is not any designated extension number (NO in StepS242), the extension number detecting section 102-1 detects theextension number of the terminal that is hooked off first (Step S243)and proceeds to Step S203 (FIG. 7).

If, on the other hand, there is a designated extension number (YES inStep S242), the extension number detecting section 102-1 detects thedesignated extension number (Step S244) and determines whether the useris present at the terminal having the extension number detected in StepS244 by referring to the information stored in the presence informationstorage section 102-17 (Step S245). If the user is absent at theterminal having the extension number (YES in Step S245), the processingoperation proceeds to Step S248. If, on the other hand, the user ispresent at the terminal having the extension number (NO in Step S245), acall-in bell of the terminal having the extension number detected inStep S244 is rung up (Step S246). If the terminal is hooked off within apredetermined period of time (NO in Step S247), the processing operationproceeds to Step S203 (FIG. 7). On the other hand, if the terminal isnot hooked off within the predetermined period of time (YES in StepS247), the processing operation proceeds to Step S248.

In Step S248, it is determined whether the current extension number hasthe lowest degree of priority by referring to the information stored inthe presence information storage section 102-17 and, if the degree ofpriority is the lowest one (YES in Step S248), the processing operationis terminated. Simultaneous call in may be used in place of terminatingthe processing operation. If the degree of priority is not the lowestone (NO in Step S248), the current extension number is switched to theextension number of the terminal having the next degree of priority byreferring to the information stored in the presence information storagesection 102-17 (Step S249) and the processing operation returns to StepS245.

Now, the processing operation of the own office side router 101 and thatof the call control device 102 at the time of automatic transferaccording to the presence information and by means of the abovedescribed second method will be described by referring to FIG. 10.

Firstly, the negotiation section 101-3 negotiates with the partneroffice side server 112 to determine the global port number to be usedfor the communication (Step S201). Then, the extension number detectingsection 102-1 determines whether an extension number is designated (StepS242). If there is not any designated extension number (NO in StepS242), the extension number detecting section 102-1 detects theextension number of the terminal that is hooked off first (Step S243)and proceeds to Step S203 (FIG. 7).

On the other hand, if there is a designated extension number (YES inStep S242), the extension number detecting section 102-1 detects thedesignated extension number (Step S244). Then, the local IP addressretrieving section 102-3 retrieves the local IP address corresponding tothe extension number that is detected in Step S244 from the extensionnumber versus local IP address correspondence table 102-2 (Step S251).Then, it is determined whether the user is present at the terminalhaving the local IP address retrieved in Step S251 by referring to theinformation stored in the presence information storage section 102-17(Step S252). If the user is absent at the terminal having the extensionnumber (YES in Step S252), the processing operation proceeds to StepS255. If, on the other hand, the user is present at the terminal havingthe extension number (NO in Step S252), a call-in bell of the terminalhaving the extension number detected in Step S244 is rung up (StepS253). If the terminal is hooked off within a predetermined period oftime (NO in Step S254), the processing operation proceeds to Step S204(FIG. 7). On the other hand, if the terminal is not hooked off withinthe predetermined period of time (YES in Step S254), the processingoperation proceeds to Step S255.

In Step S255, it is determined whether the current local IP address hasthe lowest degree of priority by referring to the information stored inthe presence information storage section 102-17 and, if the degree ofpriority is the lowest one (YES in Step S255), the processing operationis terminated. Simultaneous call in may be used in place of terminatingthe processing operation. If the degree of priority is not the lowestone (NO in Step S255), the current local IP address is switched to thelocal IP address of the terminal having the next degree of priority byreferring to the information stored in the presence information storagesection 102-17 (Step S256) and the processing operation returns to StepS252.

[Embodiment 2]

The call control device 102 acquires information for identifying therecord to be rewritten in the masquerade table 101 and then the recordis designated by the identifying information so as to rewrite the recordin the first embodiment. However, if the own office side router 101 andthe call control device 102 communicate with each other according to theuPnP (Universal Plug and Play) Standard, it is not possible to acquireinformation necessary for identifying the record describing acombination of a particular local IP address and a particular local portnumber out of the records in the masquerade table 101-1. Additionally,if the own office side router 101 and the call control device 102communicate with each other according to the uPnP Standard, it is notpossible for the call control device 102 to rewrite any record in themasquerade table 101-1.

On the other hand, if the own office side router 101 and the callcontrol device 102 communicate with each other according to the uPnPStandards, it is possible for the call control device 102 to erase arecord designated by a global port number from the masquerade table101-1 and add a record describing a desired corresponding relationshipof a combination of a global IP address and a pair of a local IP addressand a local port number to the masquerade table 101-1.

Therefore, the second embodiment is so adapted that no record in themasquerade table 101-1 is rewritten but any desired record can be erasedand a new record can be added to replace the erased record in order toachieve an effect equivalent to that of rewriting a record. Once arecord is erased, no record corresponding to the erased record existsuntil a new record is added as replacement. However, no problem arisesbecause no communication takes place between the erasure of a record andthe addition of a replacement record.

As shown in FIG. 11, the own office side router 101 of the secondembodiment has a configuration similar to that of the own office siderouter 101 of the first embodiment

As shown in FIG. 11, the call control device 102 of the secondembodiment is partly identical with the call control device 102 of thefirst embodiment but partly different from the latter. Morespecifically, the call control device 102 of the second embodiment has areplica 102-50 of the masquerade table, an extension number detectingsection 102-51, an extension number versus local IP addresscorrespondence table 102-52, a local IP address retrieving section102-53, a local port number storage section 102-54, a record addingsection 102-55, a terminal interface 102-56, a local IP addressassigning section 102-57, an origin-of-transfer-related informationdetecting section 102-58, a record retrieving section 102-59, adestination terminal designation receiving section 102-60, a transferhold receiving section 102-61, a release terminal detecting section102-62, a post-transfer local IP address detecting section 102-63, arecord erasing section 102-65, a presence information acquiring section102-66 and a presence information storage section 102-67.

The replica 102-50 of the masquerade table has a configuration same asthat of the masquerade table 101-1 and stores the records same as thoseof the latter. The replica 102-50 of the masquerade table does not havea field for global IP addresses like the masquerade table 101-1. In thefollowing description, it is assumed that there is no field for globalIP addresses.

The extension number detecting section 102-51 is similar to theextension number detecting section 102-1 of the first embodiment.

The extension number versus local IP address correspondence table 102-52is similar to the extension number versus local IP addresscorrespondence table 102-2 of the first embodiment.

The local IP address retrieving section 102-53 is similar to the localIP address retrieving section 102-3 of the first embodiment

The local port number storage section 102-54 is similar to the localport number storage section 102-4 of Embodiment 1.

At the time of signaling, the record adding section 102-55 receives theglobal IP address and the global port number to be used for the comingcommunication from the negotiation section 101-3. Note, however, that itis not necessary for the record adding section 102-55 to receive theglobal IP address from the negotiation section 101-3 if the global IPaddress of the own office side router 101 is known or if no field isprovided for global IP addresses in the masquerade table 101-1 and thereplica 102-50 of the masquerade table. Additionally, at the time ofsignaling, the record adding section 102-55 receives the local IPaddress to be used for the coming communication (the local IP address ofthe terminal to be used for the coming communication) from the local IPaddress retrieving section 102-53. Furthermore, at the time ofsignaling, the record adding section 102-55 receives the local portnumber to be used for the coming communication from the local portnumber storage section 102-54. Then, the record adding section 102-55adds a record describing the global IP address, the global port number,the local IP address and the local port number it received to themasquerade table 101-1 and the replica 102-50 of the masquerade table.Note, however, that the added record describes only the global portnumber, the local IP address and the local port number if no field isprovided for global IP addresses in the masquerade table 101-1 and thereplica 102-50 of the masquerade table.

Additionally, after the record erasing section 102-65 has erased arecord, the record adding section 102-55 subsequently operates fortransfer of communication in a manner as described below. Namely, therecord adding section 102-55 receives the global port number to bedescribed in the record to be added from the record retrieving section102-59. Additionally, the record adding section 102-55 receives thepost-transfer local IP address to be described in the record to be addedfrom the post-transfer local IP address detecting section 102-63.Furthermore, the record adding section 102-55 retrieves the local portnumber to be described in the record to be added from the local portnumber storage section 102-54 depending on the type of communication.Note, however, the local port number to be described in the record to beadded may be determined by negotiating with the destination terminal oftransfer. Then, the record adding section 102-55 adds the receivedglobal port number, the received local IP address and the retrieved ordetermined local port number to the masquerade table 101-1 and thereplica 102-50 of the masquerade table.

The terminal interface 102-56 is similar to the terminal interface 102-6of the first embodiment.

The local IP address assigning section 102-57 is similar to the local IPaddress assigning section 102-7 of the first embodiment.

The origin-of-transfer-related information detecting section 102-58 issimilar to the origin-of-transfer-related information detecting section102-8 of the first embodiment.

The record retrieving section 102-59 receives the local IP address andthe local port number of the origin of transfer from theorigin-of-transfer-related information detecting section 102-58 andretrieves the record describing the local IP address and the local portnumber from the replica 102-50 of the masquerade table. Then, the recordretrieving section 102-59 notifies the record erasing section 102-65 andthe record adding section 102-55 of the global port number described inthe retrieved record.

The destination terminal designation receiving section 102-60 is similarto the destination terminal designation receiving section 102-10 of thefirst embodiment. The transfer hold receiving section 102-61 is similarto the transfer hold receiving section 102-11 of the first embodiment.

The release terminal detecting section 102-62 is similar to the releaseterminal detecting section 102-12 of the first embodiment.

The post-transfer local IP address detecting section 102-63 is similarto the post-transfer local IP address detecting section 102-13 of thefirst embodiment.

At the time of transfer of communication, the record erasing section102-65 operates in a manner as described below. Namely, the recorderasing section 102-65 acquires the global port number, described in therecord that describes the local IP address and the local port number ofthe origin of transfer, from the record retrieving section 102-59. Then,the record erasing section 102-65 erases the record that describes theglobal port number acquired from the record retrieving section 102-59from the masquerade table 101-1 and the replica 102-50 of the masqueradetable.

At the time of end of communication, the record erasing section 102-65operates in a manner as described below. Namely, the record erasingsection 102-65 detects the extension number or the local IP address ofthe terminal that has terminated a communication. If the record erasingsection 102-65 detects the extension number of a terminal that hasterminated a communication, the record erasing section 102-65 retrievesthe local IP address corresponding to the extension number from theextension number versus local IP address correspondence table 102-52.The record erasing section 102-65 also detects the type of thecommunication that has been terminated or the local port number used forthe communication that has been terminated. If the record erasingsection 102-65 detects the type of the communication that has beenterminated, the record erasing section 102-65 retrieves the local portnumber that corresponds to the type of the communication from the localport number storage section 102-54. If, for example, the type ofcommunication is limited to IP telephony and the local port number isfixed, the record erasing section 102-65 does not have to detect norretrieve the local port number but only has to retain the local portnumber. Then, the record erasing section 102-65 requests the recordretrieving section 102-59 to retrieve the global port number describedin the record that describes the detected or retrieved local IP addressand the detected or retrieved local port number from the replica 102-50of the masquerade table and acquires the global port number retrieved bythe record retrieving section 102-59. Then, the record erasing section102-65 erases the record that describes the global port number acquiredfrom the record retrieving section 102-59 from the masquerade table101-1 and the replica 102-50 of the masquerade table. By doing so, theunnecessary port is closed to improve the security and prevent theunnecessary record from remaining in the masquerade table 101-1 and thereplica 102-59 of the masquerade table.

The presence information acquiring section 102-66 is similar to thepresence information acquiring section 102-16 of the first embodiment.

The presence information storage section 102-67 is similar to thepresence information storage section 102-17 of the first embodiment.

Next, the operation of the own office side router 101 and that of thecall control device 102 at the time of starting a communication will bedescribed below by referring to FIG. 12.

Firstly, the negotiation section 101-3 negotiates with the partneroffice side server 112 to determine the global port number to be usedfor the communication (Step S301). Then, the extension number detectingsection 102-51 detects the extension number of the terminal to be usedfor the communication (Step S302). Then, the local IP address retrievingsection 102-53 retrieves the local IP address that corresponds to theextension number detected in Step S302 from the extension number versuslocal IP address correspondence table 102-52 (Step S303). The local IPaddress is that of the terminal to be used for the communication.Thereafter, the record adding section 102-55 determines the local portnumber to be used for the communication (Step S304). Then, the recordadding section 102-55 adds a record describing a relationship betweenthe global port number determined in Step S301 and a pair of the localIP address retrieved in Step S303 and the local port number determinedin Step S304 to the masquerade table 101-1 (Step S305). Then, the recordadding section 102-55 adds a record identical with the record added tothe masquerade table 101-1 in Step S305 to the replica 102-50 of themasquerade table (Step S306).

Now, the operation of the own office side router 101 and that of thecall control device 102 of the second embodiment 2 at the time oftransferring the communication will be described below by referring toFIG. 13.

Firstly, the origin-of-transfer-related information detecting section102-58 detects the extension number of the origin terminal of transfer(Step S311). Then, the origin-of-transfer-related information detectingsection 102-58 retrieves the local IP address that corresponds to theextension number detected in Step S311 from the extension number versuslocal IP address correspondence table 102-52 (Step S312). Alternatively,the origin-of-transfer-related information detecting section 102-58 maydirectly detect the local IP address of the origin terminal of transferin place of the detecting operation of Steps S311 and S312 (Step S313).Then, the origin-of-transfer-related information detecting section102-58 detects the local port number that is being used for thecommunication (Step S314). Thereafter, the record retrieving section102-59 retrieves the global port number described in the record thatdescribes the local IP address retrieved in Step S312 or detected inStep S313 and the local port number detected in Step S314 from thereplica 102-50 of the masquerade table (Step S315). Then, the recorderasing section 102-65 erases the record describing the global portnumber retrieved in Step S315 from the masquerade table 101-1 (StepS316) and the record describing the global port number retrieved in StepS315 from the replica 102-50 of the masquerade table (Step S317).

Subsequently, the destination terminal designation receiving section102-60 or the release terminal detecting section 102-62 detects theextension number of the destination terminal of transfer (Step S318).Then, the post-transfer local IP address detecting section 102-63retrieves the local IP address that corresponds to the extension numberof the destination of transfer detected in Step S318 from the extensionnumber versus local IP address correspondence table 102-52 (Step S319).Alternatively, the destination terminal designation receiving section102-60 or the release terminal detecting section 102-62 may directlydetect the local IP address of the destination terminal of transfer inplace of the detecting operation of Steps S318 and S319 (Step S320).Then, the record adding section 102-55 determines the post-transferlocal port number (Step S321). Then, the record adding section 102-55adds a record describing the global port number retrieved in Step S315,the local IP address retrieved in Step S319 or the local IP addressdetected in Step S320 and the local port number determined in Step S321to the masquerade table 101-1 (Step S322) and a record describing thesame contents as the record added in Step S322 to the replica 102-50 ofthe masquerade table (Step S323).

Next, the processing operation of the own office side router 101 andthat of the call control device 102 of Embodiment 2 at the time ofautomatic transfer according to the presence information and by means ofthe above described first method will be described by referring to FIG.14.

It will be clear by comparing FIG. 9 and FIG. 14 that a processingoperation same as that of the first embodiment takes place in the secondembodiment. Note, however, that the processing operation proceeds toStep S303 (FIG. 12) after Step S243 if the outcome of determination inStep S242 is NO and proceeds to Step S303 (FIG. 12) if the outcome ofdetermination in Step S247 is NO.

Now, the processing operation of the own office side router 101 and thatof the call control device 102 of the second embodiment 2 at the time ofautomatic transfer according to the presence information and by means ofthe above described second method will be described by referring to FIG.15.

It will be clear by comparing FIG. 10 and FIG. 15 that a processingoperation same as that of the first embodiment takes place in the secondembodiment 2. Note, however, that the processing operation proceeds toStep S303 (FIG. 12) after Step S243 if the outcome of determination inStep S242 is NO and proceeds to Step S304 (FIG. 12) if the outcome ofdetermination in Step S254 is NO.

To have the person at the partner office hear a melody representinghold, it is only necessary to assign a local IP address and a local portnumber to a sound source of the melody representing hold and transfer itto the local IP address and the local port number.

The present invention is applicable to transfer of IP communications.

What is claimed is:
 1. A communication transfer apparatus comprising: apresence information storage section which stores information onpresence or absence of a user at each terminal and information onpriorities of terminals for each of users, wherein said terminals foreach of users are located in a local IP network to which a common globalInternet Protocol address is assigned; a masquerade table including aplurality of records, on each of which recorded is a global InternetProtocol address common among said plurality of records, a global portnumber, a local Internet Protocol address, and a local port number; acalled terminal detecting section which detects a called terminal basedon signals received via a global IP network or outputs fnmn saidterminals; a presence determining section which determines presence orabsence of a user at the called terminal detected by said calledterminal detecting section by referring to the information on presenceor absence of said user at the detected called terminal stored in saidpresence information storage section; a transfer destination identifyingsection which identifies a terminal having a next priority by referringto the information on priorities a terminals for said user at thedetected called terminal stored in said presence information storagesection if said presence determining section determines absence of auser at the called terminal detected by said called terminal detectingsection; and a record rewriting section which retrieves a record whichcorresponds to the called terminal from said masquerade table includingsaid plurality of records and changes a local Internet Protocol addressof the retrieved record into a local Internet Protocol address of theterminal identified by said transfer destination identifying section,wherein two or more transfers take place with respect to two or moredifferent called terminals located in the local IP network to which thecommon global Internet Protocol address is assigned.
 2. Thecommunication transfer apparatus according to claim 1, wherein saidrecord rewriting section changes the local Internet Protocol addresswhich corresponds to said called terminal without changing the globalport number which corresponds to said called terminal in said masqueradetable.
 3. The communication transfer apparatus according to claim 1,wherein said transfer destination identifying section relates as pair ofsaid local Internet Protocol address and said local port number not onlyto said global Internet Protocol address but also to a global portnumber in said masquerade table.
 4. A communication transfer apparatuscomprising: a priority storing section which stores information onpriorities of IP terminals for each of users, wherein said IP terminalsfor each of users are located in a local IP network to which a commonglobal Internet Protocol address is assigned; a masquerade tableincluding, a plurality of records, on each of which recorded is a globalInternet Protocol address common among said plurality of records, aglobal port number, a local Internet Protocol address, and a local portnumber; a called IP terminal detecting section which detects a called IPterminal based on signals received via as global IP network or outputsfrom said IP terminals; a presence determining section which determinesexecution or non-execution of a hooking off operation at the called IPterminal detected by said called IP terminal detecting section within apredetermined period of time:, a transfer destination identifyingsection which identifies an IP terminal having a priority next to apriority of the called IP terminal by referring to the information onpriorities of IP terminals for said user at the detected called IPterminal stored in said priority storing section if said presencedetermining section determines non-execution of the hooking offoperation at the called IP terminal detected by said called IP terminaldetecting section within the predetermined period of time, wherein thetransfer destination identifying section is disposed in a local IPnetwork where said called IP terminal and said identified IP terminalare disposed; and a record rewriting section which retrieves a recordwhich corresponds to the called IP terminal from said masquerade tableincluding said plurality of records and changes a local InternetProtocol address of the retrieved record into a local Internet Protocoladdress of the IP terminal identified by said transfer destinationidentifying section, wherein two or more transfers take place withrespect to two or more different called IP terminals located in thelocal IP network to which the common global Internet Protocol address isassigned.
 5. A communication transfer method comprising: a presenceinformation storing step of storing information on presence or absenceof a user at each terminal and information on priorities of terminalsfor each of users to a record medium, wherein said terminals for each ofusers are located in a local IP network to which a common globalInternet Protocol address is assigned; a masquerade table storing stepof storing a masquerade table including a plurality of records, on eachof which recorded is a global Internet Protocol address common amongsaid plurality of records, a global port number, a local InternetProtocol address, and a local port number; a called terminal detectingstep of detecting a called terminal based on signals received via aglobal IP network or outputs from said terminals; a presence determiningstep of determining presence or absence of a user at the called terminaldetected by said called terminal detecting step by referring to theinformation on presence or absence of said user at the detected calledterminal stored in said record medium; a transfer destinationidentifying step of identifying a terminal having a next priority byreferring to the information on priorities of terminals for said user atthe detected called terminal stored in said record medium if saidpresence determining step determines absence of a user at the calledterminal detected by said called terminal detecting step; and a recordrewriting step of retrieving a record which corresponds to the calledterminal from said masquerade table including said plurality of recordsand changing a local Internet Protocol address of the retrieved recordinto a local Internet Protocol address of the terminal identified bysaid transfer destination identifying step, wherein two or moretransfers take place with respect to two or more different calledterminals located in the local IP network to which the common globalInternet Protocol address is assigned.
 6. A communication transfermethod comprising: a priority storing step of storing information onpriorities of IP terminals for each of users to a record medium, whereinsaid IP terminals for each of users are located in a local IP network towhich a common global Internet Protocol address is assigned; amasquerade table storing step of storing a masquerade table including aplurality of records, on each of which recorded is a global InternetProtocol address common among said plurality of records, a global portnumber, a local Internet Protocol address, and a local port number; acalled IP terminal detecting step of detecting a called IP terminalbased on signals received via a global IP network or outputs front saidIP terminals; a presence determining step of determining execution ornon-execution of a hooking off operation at the called IP terminaldetected by said called IP terminal detecting step within apredetermined period of time; a transfer destination identifying step ofidentifying an IP terminal having a priority next to a priority of thecalled IP terminal by referring to the information on priorities of IPterminals for said user at the detected called IP terminal stored insaid record medium if said presence determining step determinesnon-execution of the hooking off operation at the called IP terminaldetected by said called IP terminal detecting step within thepredetermined period of time, wherein the transfer destinationidentifying section is disposed in a local IP network where said calledIP terminal and said identified IP terminal are disposed; and a recordrewriting step of retrieving a record which corresponds to the called IPterminal from said masquerade table including said plurality of recordsand changing a local Internet Protocol address of the retrieved recordinto a local Internet Protocol address of the IP terminal identified bysaid transfer destination identifying step, wherein two or moretransfers take place with respect to two or more different calledterminals located in local IP network to winch the common globalInternet Protocol address is assigned.